The invention relates to a projection display device comprising an illumination system having a light source and an optical guiding means for providing an illumination beam, color filter means being provided on a wheel, for transmitting colored portions of the illumination beam, and an image display system having a display panel for modulating the colored portions of the illumination beam with image information and projecting the image on a screen.
Projection display devices can be used in both rear and front image projection systems. In a rear projection system, the projection display device projects an image representing television or datagraphic information on the rear side of a diffusing transparent screen, which front side is directed to a viewing audience. In a front projection system, the projection display device projects an image representing television or datagraphic information on the front side of a reflecting screen, which front side is directed to a viewing audience.
Such a projection display device is known from U.S. Pat. No. 6,005,722. The known projection system comprises an illumination system and an image display system having an image display panel for modulating an illumination beam to be supplied by the illumination system with image information. The illumination system comprises a light source and an integrator system for forming an illumination beam. A color wheel is present between the light source and the integrator system. The color wheel includes red, green and blue filters. The color filters occupy angular segments of the color wheel. The color wheel filters the white light from the illumination system to produce a colored illumination beam that changes colors from red to green, from green to blue and from blue to red and so on in a rate proportional to the angular speed of the wheel. The display panel modulates the colored illumination beam in accordance with corresponding image information of the color of the illumination beam incident on the display panel. A reflective polarizer may be present between the color filter wheel and the display system. The reflective polarizer transmits a portion of the illumination beam having a polarization in a first direction and reflects a portion of the illumination beam having a second polarization in a second direction, perpendicular to the first direction. Furthermore, light recycling means may be present in the projection display device to reuse the reflected light from the reflective polarizer. Dichroic filters can also be used in the color filter wheel. However, in the known projection device, the application of the dichroic color filter wheel does not improve the light efficiency of the projection display device.
It is an object of the invention to provide a projection display device having an increased light efficiency. This object can be achieved by the projection display device in accordance with the invention, and is characterized in that the color filter means comprises adjacent spiral-shaped filter portions for simultaneously illuminating portions of the display panel by the colored portions of the illumination beam so that the portions of the display panel are consecutively illuminated by the colored portions of the illumination beam. In the device as claimed, the unused portion of the illumination beam can be fed back in the illumination system. The recuperated light can then be recycled in the illumination system and used again in the illumination beam. The invention is based on the recognition that a projection display device with a single display panel and the known configuration of the color filter wheel makes it impossible to recycle a reflected portion of the illumination beam of the dichroic filters because this reflected portion of the illumination beam has undesired colors which cannot be used again. In the new arrangement, the filter portions provided on the wheel are arranged in such a way that the display panel is simultaneously illuminated with an illumination beam having three portions of different colors, while the filtered portion of the illumination beam corresponding to one colored portion of the color filter can be used to increase the light output in the illumination beam for the other colored portions. For example, the red filter transmits red light of the illumination beam and reflects green and blue light. This can be used to increase the light intensity of the green and blue portions of the illumination beam.
In a particular advantageous embodiment, the spiral-shaped portions extend from a first radius to a second radius of the wheel, the second radius being larger than the first radius, in which the first and second radii depend on the size of the display panel.
In a further embodiment of the projection device according to the invention, the color filter portions have an identical shape.
In this embodiment, the illumination of the display panel for each color is equal. In a further embodiment of the projection display device according to the invention, the spiral-shaped filter portions are arranged to transmit red, green and blue light, respectively.
A further embodiment of the invention according to the invention is characterized in that the filter portions are arranged to transmit red, green, blue and white light, respectively. Adding a white segment allows a higher brightness of the projected image in exchange for a reduced color saturation.
A further embodiment of the invention according to the invention is characterized in that a cross-section in a radial direction of a spiral-shaped portion of a color selected from the group of red, green, blue and white extends from the first radius to the second radius of the wheel. This embodiment allows synchronization at the instant when the display panel is completely illuminated with white light. This results in a reduced visibility of the interference of a light source synchronization pulse and the color scanning. If an UHP light source is applied, the light source is supplied with a square-wave current source. A known drawback of such a light source is that the stability of the arc is not constant in time. In order to reduce this variation of the arc, a current pulse is superimposed on the square-wave current for the light source.
A further embodiment of the projection display device according to invention is characterized in that the projection display device is provided with a control means for providing the light source with a square-wave current for supplying a constant power to the light source and for imposing an arc-stabilisation pulse on the square-wave current for each change of polarity of the square-wave current, the control means being arranged to synchronise the occurrence of the arc-stabilisation pulse with the occurrence in the illumination beam of a maximum cross-section in the radial direction of the spiral-shaped portion having the selected color. In order to reduce the visibility of interference, the current pulse is synchronized with the rotation of the color wheel.
A further embodiment of the projection display device according to the invention is characterized in that the spiral-shaped portions comprise a dichroic filter. Use of a dichroic filter allows the use of light recycle means recuperating a portion of the illumination beam not having the desired colors reflected from the color filter.
A further embodiment of the projection display according to the invention is characterized in that the color filter wheel comprises a cholesteric color filter. Cholesteric color filters are known per se from the published international application WO 00/34808. In this application, a method of manufacturing a cholesteric filter is also described. This method can also be applied for manufacturing the color filter wheel.
A further embodiment of the projection display device according to the invention is characterized in that the projection display device is provided with a transmissive image display panel or a reflective image display panel.